1. Field of the Invention
The present invention relates to a method for molding an optical element in which the optical element having a complex face shape such as an aspherical lens is press-molded at high precision.
2. Related Background Art
Recently, as optical instruments become smaller and lighter, it is desired to reduce the number of glass lenses for use in the optical system. One means for realizing this includes using an aspherical lens capable of correcting aberration even if the number of lenses is reduced. A method of producing such a lens having an aspherical shape is well known, in which glass material is sandwiched between the mold members having a predetermined surface precision and then is press molded.
A conventional method for molding an optical element by press molding has been disclosed in Japanese Patent Publication No. 61-32263. This method is such that glass material is sandwiched between a pair of mold members having a molding face finished to a face shape precisely corresponding to an ideal form of a completed shape of an optical element, and is press-molded in a range of temperatures at which the viscosity of glass material is from 10.sup.8 to 5.times.10.sup.10 poise. Thereafter, the cooling is effected so that the temperature difference of the glass material and the mold members may not exceed at least 20.degree. C., and a molded optical element is taken out from the mold members in a range of temperatures at which the viscosity of glass material is less than 10.sup.12 poise. With such a method, it is possible to produce high precision optical elements.
In the above conventional example, however, when producing an optical element of a shape whose surface accuracy is hardly obtained, such as a concave lens having a large radius of curvature of the surface or a meniscus lens, it often occurs that required surface accuracy (e.g., a high accuracy value such as less than one-fourth (1/4) line of Newton ring) can not be satisfied even though various molding conditions are set to be optimum.
In order to improve the surface accuracy of a completed optical element even in a minor way, the pressing force is necessary to be strictly controlled in the cooling step after molding, for example, but it is quite difficult to control the pressing pressure strictly. Also, any slight change in the other molding conditions will decrease the surface accuracy. Furthermore, to improve the surface accuracy of an optical element, an auxiliary device may be often needed, so that there arises a problem that the cost of a processing apparatus is increased, thus giving rise to the higher cost of the optical element itself.